1. Field of the Invention
The present invention relates to a position detection apparatus for detecting a position of a movable portion in a plane using a magnetoresistive effect element and a magnet.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication Nos. 2005-331401, 2005-69744 and 2004-69695 as below disclose the position detector which employs the magnet and the detection unit for detecting the magnetic field generated by the magnet.
Each of the position detector disclosed in the aforementioned documents has the detection unit provided with the element capable of detecting intensity of the magnetic field such as a hall element. The intensity of the magnetic field generated from the surface of the magnet, which is directed orthogonally to the surface is detected by the detection unit to locate an opposed position between the magnet and the detection unit.
The detection unit as disclosed in the aforementioned documents serves to detect only the magnetic field intensity perpendicular to the magnet surface such as the hall element, thus restricting the region which allows the detection unit to identify the position. When the magnet has the upper surface magnetized to N-pole and the back surface magnetized to S-pole, and the detection unit is oppositely positioned to the upper surface, the change in the intensity of the magnetic field perpendicular to the upper surface upon movement of the detection unit away from the center of the magnet surface to one side is the same as the change upon movement of the detection unit away from the center to the other side. The magnetic field intensity may be detected by the detection unit which moves while passing the center. However, the position cannot be identified as to which side the position is located with respect to the center.
As a result, the region which allows the detection unit to detect the position becomes considerably narrow, and the magnet has to be made large for the purpose of accurately detecting the position of the detection unit on the X-Y coordinate in the wider region. The use of the hall element for the detection unit may complicate the circuit for forming the detection unit.
Japanese Unexamined Patent Application Publication No. 2006-276983 discloses the magnetic sensor for detecting the magnetic field generated by the circular magnet when it moves. The magnetic sensor is provided with four huge magnetoresistive effect elements, two of which detect X-direction component of the magnetic flux from the magnet, and the other two of which detect Y-direction component of the magnetic flux from the magnet. The difference of the change in the resistance values between the two huge magnetoresistive effect elements for detecting the X-direction component of the magnetic flux, and the difference of the change in the resistance values between the other two huge magnetoresistive effect elements for detecting the Y-direction component of the magnetic flux are obtained, respectively to locate the position of the magnet.
Each of the huge magnetoresistive effect elements is incapable of distinguishing between the oppositely directed magnetic fluxes, that is, the positive and the negative directions. It is therefore difficult to accurately identify the movement position of the magnet.
In the case where the two huge magnetoresistive effect elements for detecting the X-direction component of the magnetic flux move away from the center toward the Y-direction, not only the X-direction component of the magnetic field but also the Y-direction component of the magnetic field are applied to the free magnetic layer of the huge magnetoresistive effect element. Accordingly, magnetization of the free magnetic layer becomes unstable, which prevents the accurate detection of the resistance change corresponding to the movement position of the magnet toward the X-direction.